A monoclonal antibody (MAb), designated B72.3, has been generated using membrane enriched fractions of a metastatic human breast carcinoma as the immunogen. Previous studies have demonstrated that the reactive antigen, a novel high molecular weight glycoprotein comples, can be detected in Formalin-fixed, paraffin-embedded tissue sections of human breast and colon carcinomas, and not in a variety of normal adult huamn tissues. We have now determined that MAb B72.3 may be used as an adjunct for diagnosis of adenocarcinoma in cytologic preparations of human effusions. Using the avidin-biotin complex method of immunoperoxidase staining and Formalin-fixed, paraffin-embedded cell suspensions, MAb B72.3 detected adenocarcinoma cells in effusions from all of 21 patients with adenocarcinoma of the breast. No reactivity was demonstrated in any cell type in benign effusions from 24 patients without cancer, or 13 patients with prior or extant cancer in other body sites; moreover, B72.3 showed no reactivity to leukemic or lymphomatous effusions, or apparent mesothelial cells from malignant effusions. Monoclonal antibody B72.3 also detected adenocarcinoma cells in cytologic effusion specimens from 12/12 patients with adenocarcinoma of the lung and 16/16 patients with adenocarcinoma of the ovary. Thus, these data suggest that the immunocytological application of MAb B72.3 should now be considered as an adjunct in the discrimination of adenocarcinoma cells from reactive mesothelial cells in the cytologic diagnosis of malignant effusions and may be applicable in the detection of occult tumor cells in needle aspirates as well as other biologic fluids.